This invention relates to variable transconductance elements and, more specifically, to switched-capacitor conductance-control of such elements to render them substantially temperature-insensitive.
It is known to make large-scale-integrated (LSI) circuits on a single semiconductor chip by utilizing standard metal-oxide-semiconductor (MOS) fabrication techniques. In the MOS technology, dense arrays of field-effect-transistor (FET) devices and high-quality small-valued capacitors are easily realized. But conventional diffused or polysilicon resistance elements formed in an MOS chip typically consume an undesirably large part of the available chip area. Moreover, such elements are temperature sensitive. As the temperature of the chip changes, the resistance values of these elements also change. In turn, such temperature-caused changes deleteriously affect the operating characteristics of LSI circuits such as high-precision active filters.
It is further known that a MOSFET device can be used as a voltage-controlled resistor for small signals of either polarity. But the resistance of such a device also varies with temperature. Thus, even for small-signal operation, MOSFET devices as heretofore proposed are not suitable for utilization in high-precision applications where resistors characterized by substantial insensitivity to temperature variations are required.
An attractive approach is available for providing small-area temperature-insensitive resistance elements in MOS chips. This approach is based on emulating resistive behavior by utilizing switched-capacitor techniques. The application of these techniques to, illustratively, the design of high-precision active filters is well known, as described, for example, by R. W. Brodersen, P. R. Gray and D. A. Hodges in "MOS Switched-Capacitor Filters," Proceedings of IEEE, Vol. 67, pages 61-75, January 1979. The operating characteristics of such filters are determined by highly stable crystal-controlled clock frequencies and capacitor ratios.
The temperature coefficient of an MOS capacitor is typically exceedingly low. And it is known that the temperature coefficient of capacitor ratios is even lower. In practice, the variation with temperature of MOS capacitor networks is thus so low as to be insignificant in almost all applications. Accordingly, switched-MOS capacitors are an advantageous basis for realizing high-precision LSI circuits that are substantially temperature-insensitive.
A circuit such as a switched-capacitor filter is in effect a sampled-data network. Accordingly, signals applied thereto must first be band-limited. This is done, for example, by utilizing a so-called antialiasing filter which is a filter of the continuous-signal type. Hence, the antialiasing filter is representative of LSI circuits that cannot be realized utilizing switched-capacitor techniques. For such circuits, therefore, a need exists for some way other than the switched-capacitor approach for implementing small-area temperature-insensitive resistors.
Moreover, although the aforespecified switched-capacitor techniques as applied, for example, to filtering are advantageous in the audio-frequency range, the use of such techniques at higher frequencies becomes more difficult and may lead to an undesirably high level of switching noise. Thus, especially for operation at higher frequencies, a need exists for a low-noise substitute for switched-capacitors in important applications such as filtering. For such applications, a small-area temperature-insensitive continuous transconductance element would obviously be a highly advantageous component.